The field of this invention is that of operating blowout preventers in deep water operations to shear pipe within the well bore and to seal the well bore to protect the environment in emergency situations when an obstruction is in the well bore.
Shear rams in blowout preventers are designed to cut the pipe in emergency situations and allow the bore to be closed and secure the well against unintended discharges of oil or gas to the environment. Blind shear rams are designed to both shear the pipe and seal off the well bore in a single movement.
As drill pipe becomes larger in diameter, greater in wall thickness, and higher in yield strength they become more and more difficult to shear. A further complication to this is that when accumulators are used as the power source to do the shearing, less than full accumulator pressure is available when the accumulators discharge enough fluid to move forward and initiate the shearing action. The combination of more difficult to shear pipe and less than full pressure to do the shearing has resulted in the fact that some of the pipes cannot be sheared.
Additional contemporary requirements are that pipe must be sheared with the maximum anticipated pressure in the bore of the BOP Stack. This pressure as high as 15,000 p.s.i. acts on the end of the operating rod pushing the shear rams to the shearing position, cancelling some of the force which was available for the shear rams.
Blowout preventer systems containing these shear rams are major pieces of capital equipment landed on the ocean floor in order to provide a conduit for the drill pipe and drilling mud while also providing pressure protection while drilling holes deep into the earth for the production of oil and gas. The typical blowout preventer stacks have an 18¾ inch bore and are usually of 10,000 or 15,000 psi working pressure. The blowout preventer stack assembly weighs in the range of five hundred to eight hundred thousand pounds. It is typically divided into a lower blowout preventer stack and a lower marine riser package.
The lower blowout preventer stack includes a connector for connecting to the wellhead at the bottom on the seafloor and contains several individual ram type blowout preventer assemblies, which will close on various pipe sizes and in some cases, will close on an open hole with what are called blind rams. Characteristically there is an annular preventer at the top, which will close on any pipe size or close on the open hole.
The lower marine riser package typically includes a connector at its base for connecting to the top of the lower blowout preventer stack, it contains a single annular preventer for closing off on any piece of pipe or the open hole, a flex joint, and a connection to a riser pipe which extends to the drilling vessel at the surface.
The purpose of the separation between the lower blowout preventer stack and the lower marine riser package is that the annular blowout preventer on the lower marine riser package is the preferred and most often used pressure control assembly. When it is used and either has a failure or is worn out, it can be released and retrieved to the surface for servicing while the lower blowout preventer stack maintains pressure competency at the wellhead on the ocean floor.
The riser pipe extending to the surface is typically a 21 inch O.D. pipe with a bore larger than the bore of the blowout preventer stack. It is a low pressure pipe and will control the mud flow which is coming from the well up to the rig floor, but will not contain the 10,000-15,000 psi that the typical blowout preventer stack will contain. Whenever high pressures must be communicated back to the surface for well control procedures, smaller pipes on the outside of the drilling riser, called the choke line and the kill line, provide this function. These will typically have the same working pressure as the blowout preventer stack and rather than have an 18¾-20 inch bore, they will have a 3-4 inch bore. There may be additional lines outside the primary pipe for delivering hydraulic fluid for control of the blowout preventer stack or boosting the flow of drilling mud back up through the drilling riser.
The blowout preventers are operated or closed in response to an electric signal from the surface to an electro-hydraulic control valve which directs fluid stored under pressure in accumulator bottles to the operating cylinders on the blowout preventer. The accumulators are set to or regulated down to the manufacturer's maximum rated working pressure of the hydraulic cylinders which are to provide the force to operate the rams, typically 5000 p.s.i. on deepwater rigs. These pressure regulators presume a large supply of fluids and regulate the pressure downstream of the regulator. Any number of events can prevent this sequence from occurring such as failure in the surface controls to send the signal, failure in the connecting lines from the surface to depth as great as 12,000′, failure of the electro-hydraulic valve to close, and absence of fluid stored under pressure.
All subsea blowout preventers have 100% redundant control systems to minimize the risk of non-operation. They are very characteristically called the yellow system and blue system and represent primary and secondary means to operate any function on the blowout preventer stack.
When all else fails, it is not necessary to have emergency operation of multiple components in the subsea blowout preventer stack. A single component—the blind shear rams can immediately secure an uncontrolled flow of oil or gas from the well. A flat faced gate from each side will meet at the middle to seal off the bore. If a pipe of any sort is in the bore at the time, it will simply shear the pipe in half and then seal. The blind shear ram is the ultimate safety device, but it must operate. Unfortunately, contemporary rams will not shear every kind of pipe in half, but are rather limited to shearing the smaller drill pipe bodies. Larger cross section and higher strength materials provide limitations on contemporary devices, providing situations in which the safety devices simply will not close.
The need to be able to send a single command which will quickly secure the well bore against discharges to the environment has long been known in the industry as indicated by a test demonstration of shearing a drill collar at the Offshore Technology Conference in Houston more than 20 years ago. Since this demonstration of the desire for this to be accomplished, manufacturers have not accomplished this, but rather have settled back in a mode of building systems which in some cases will shear only the drill pipe body and the tool joint, and in some cases the products offered will only shear the drill pipe body and will not shear the drill pipe tool joint. The need for this level of safety has long been known, and industry has simply not figured out how to practically achieve this.